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American hostage Peter Kassig beheaded by ISIS militants in new video released online

Gerges: Kassig's beheading reveals a terror group that is desperate and on the run

Gerges: American-led airstrikes are working and ISIS' leaders are being driven underground

Gerges: Sunni tribes that were loyal to ISIS are now peeling off amid the group's ongoing brutality

The beheading of American aid worker Peter Kassig by ISIS is an act of cruelty, but it's also an act of desperation by a terrorist organization that has found itself on the run.

None of us, sadly, can be surprised anymore at the brutality of ISIS, the insidious group that now controls huge chunks of territory in Syria and Iraq. But Kassig's murder also reveals how little regard the so-called "Islamic State" has for Islamic values.

Moderate and radical Islamic clerics alike had called for ISIS to spare the life of Kassig -- also known as Abdul-Rahman -- a former U.S. Army Ranger who toured Iraq in 2007 and converted to Islam during his year in captivity.

Killing a convert to Islam is an extremely serious violation of the well-established consensus in the Islamic community on the sacredness of life for converts to the religion.

Fawaz Gerges

Abu Muhammed al-Maqdsi, a mentor to many al Qaeda leaders, had called for mercy -- not only because Kassig was a convert to Islam, but because he had given up so much to move to Syria and help victims of the war. Militant Islamists in the country also went public with a request for mercy. They said Kassig, a trained medic, had treated them when they were injured in battles against Syrian government forces.

It was inevitable that these calls would fall on deaf ears. Beheading Western hostages is one of the only tools ISIS has at its disposal to retaliate against the American-led airstrikes that are beginning to land serious blows on the group. Hundreds of fighters have been killed by airstrikes in Iraq and Syria, particularly around the Syrian town of Kobani, and Iraqi troops have also made a series of advances.

It took the Americans about two months to build up a critical mass of intelligence -- what U.S. military officials call "patterns of life" -- about the behaviors and organization of ISIS, but in the last few weeks that critical work has begun to bear fruit.

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On November 7, American warplanes bombed a convoy of vehicles near Mosul, in northern Iraq. The airstrikes were conducted against what was assessed to be a meeting of ISIS leaders near the town. Unconfirmed reports suggested that a number of top commanders -- including ISIS chief Abu Bakr al-Baghdadi -- could have been injured in the attack.

While it is difficult to keep track of the latest developments on the ground, what we do know is this: The momentum, at least in Iraq, is shifting. The group's leaders are being hunted down, and they're feeling the pain.

In July, a video emerged of a defiant Baghdadi delivering a sermon at a mosque in Mosul. Last week, ISIS released an audio clip of a man purporting to be Baghdadi -- a sign that, like al Qaeda's leaders before them, ISIS bosses are going deeper and deeper underground.

ISIS' surge has been blunted by U.S. airstrikes. The days when the group could sweep into a town and take it over seem long gone. The Iraqi army along with its allies -- the Kurds and the Shia militias -- have made good progress in the past two months, retaking Baiji and other strategic towns in Iraq. The nation's military is being restructured and rebuilt, and the Iraqi state is beginning to stand on its own two feet.

The terror group's success up until now has been based on its ability to sell its narrative to poor local Sunni communities. ISIS has portrayed itself as the defender of persecuted Sunnis against sectarian Shia-led governments in Iraq and Syria and elsewhere, and embedded itself into the communities of cities like Mosul, Tikrit and Fallujah.

This narrative brought thousands of Sunnis together to fight the Shia under ISIS' banner, even though these tribes didn't subscribe to the group's extremist ideology. But more and more Sunnis are beginning to realize that ISIS isn't their friend. In fact Muslims, have until now borne the brunt of ISIS' brutality. The group has killed thousands of Shias and Sunnis are also being targeted.

In August, ISIS fighters massacred 700 members of the Sunni Shaitat tribe in eastern Syria, according to the Washington Post. In early November, ISIS murdered hundreds of members of the Albu Nimr tribe, a powerful Sunni clan in Anbar province, west of Baghdad. Today, more and more Sunni tribes are beginning to organize themselves in opposition to ISIS' bloody reign.

The battle for Kobani could now prove to be the turning point. ISIS has made a last stand of sorts in this small Kurdish town on Syria's border with Turkey. The group wanted to send a message to the world that not only could it defeat local opposition, it could defeat the Americans. That mission has failed, and it was a monstrous strategic error.

ISIS poured thousands of skilled fighters into Kobani. We have evidence that many of them never returned, and that ISIS has forced local policemen to join their ranks.

But the heroic will of the Kurdish defense forces in Kobani have broken the group's will -- and most significantly, they have shattered ISIS' narrative of invincibility.

Kobani has shown that ISIS can be defeated. ISIS has been stopped in its tracks. The group is bleeding and the beheading of Kassig shows ISIS' desperation to lash out, to show that it still has options.

Is the battle over? Of course not. It will be long, it will be costly, it will be risky, and it will last for years -- perhaps even a decade. This is an army of tens of thousands of skilled, fanatical soldiers who are willing to kill and be killed. The voice (purported to be Baghdadi's) on last week's recording declared: "They will never abandon fighting ... They will be triumphant even if only one man of them is left."

The top-down approach of airstrikes and ground forces won't rid the Middle East of ISIS' insidious ideology. This is not about winning one or two battles -- it's about dismantling Baghdadi's killing machine from the ground up. The group can only be defeated when its sectarian narrative about the persecution of Sunnis is dismantled.

We need a bottom-up approach that addresses the legitimate grievances of poor Sunni communities in Iraq and Syria, and gives them a stake in their political future. We need a political solution to the civil war in Syria that has provided ISIS with motivation, funding, and thousands of skilled soldiers. And we need to put an end to the proxy war being waged in these countries by the Sunni rulers of Saudi Arabia and the Shia regime in Iran.

Given the complexity of the situation, this is all easier said than done. The latest developments have shown that ISIS is not invincible -- but to defeat the group, military cooperation without true political consensus on how to end the crisis will not be enough.